The present invention is a fibrous cellulosic product containing a uniformly dispersed polymeric material which has been deposited in an aqueous suspension from an anionic latex. The invention further comprises the method of making the products. These products are especially advantageous for making air laid webs wherein the polymer serves as a heat activatable bonding agent.
Treatment of cellulosic products with polymers of various types has a long history in the pulp and papermaking art. Depending on the particular polymeric system being used, and the ultimate effect desired, this treatment may take place either before or after formation of the sheet at the wet end of a paper machine. Often it is desired to retain the polymer on or near the surface or surfaces of the sheet. In this case, it can be applied by any of the conventional coating techniques. For other applications, it is desirable for the polymer to be distributed uniformly throughout the sheet. Where large amounts of polymer are desired, this can be accomplished by dipping or impregnating the sheet after the papermaking process. However, where uniform distribution of smaller amounts of polymer is desired, it is usually preferred to include the additive with the stock prior to the papermaking process. Unfortunately, this is not always possible. Many polymeric materials must be used in the form of aqueous emulsions. These emulsions are usually anionic in nature and, almost universally, will have water as the continuous phase. Within the papermaking industry, these polymer emulsions are typically referred to as "latexes or latices." In the present application the term "latex" refers to very broadly to any anionic aqueous emulsion of a polymeric material. These polymers can range from hard vitreous types to those which are soft and rubbery. They may be either thermoplastic or thermosetting in nature. In the case of thermoplastic polymers they may be materials which remain permanently thermoplastic or they may be types which are partially or fully crosslinkable, with or without an external catalyst, into thermosetting types.
Because of their anionic nature, very few latices can be added directly to a pulp making slurry with the expectation of having satisfactory retention. The cellulosic fibers are also anionic and they will repel the resin particles unless the fiber surface is modified in some means to make it less negative in character. Cationic retention aids are sometimes used to accomplish this purpose. Examples of this practice are found in recent U.S. Pat. Nos. to Jukes, et al., 4,125,645 and 4,256,807. A paper by Latimer and Gill, Tappi 56(4): 66-69 (1973), describes the beater deposition of an acrylic latex onto wood pulp using a cationic deposition aid. Another approach outlined in Japanese Kokai 85,374/74 has been to create a cationic latex. However, this approach is possible with only a very limited number of polymeric materials.
The use of cationic retention or deposition aids is not without problems in its own right. Retention aids tend to be quite expensive and any given retention material may be totally ineffective with the latex of choice. Rarely do retention efficiencies exceed 60-70%. For these reasons, it has not been the usual practice to date to employ wet addition of latices except in very selective circumstances.
Another approach has been to precipitate the polymer particles on the fibers by pH change or by chemical additives. This method can cause the latex to agglomerate and form relatively large globules rather than producing a uniform fiber coating.
One problem with the use of retention aids has been the inability of the papermaker to precisely control the electrical charge of the fibers. An approach that has received some study over the years has been to chemically modify the fiber surface to make it less negative. Uwatoko, Kagaku Kogyo (Japan) 25(3): 360-362 (1974), briefly summarizes the state of art in regard to cationic fibers and lists six major approaches that have been taken. The first method introduces side chains containing a tertiary nitrogen atom. These side chains are attached to the cellulose molecule at the hydroxyl groups as ethers. One product of this type which has received considerable study is the quaternized diethylaminoethyl derivative of cellulose. A second route to the preparation of cationic cellulose is the reaction of cellulose in the presence of sodium hydroxide with ethanolamine, aqueous ammonia, or melamine. A third process is the reaction between cellulose and a material such as 2-aminoethyl sulfuric acid in the presence of sodium hydroxide. Another product has been formed by iminating an aminated cellulose by reaction between the aminated cellulose and ethylene imine. An approach which has received considerable study is the reaction of various trimethyl ammonium salts. Of particular importance has been glycidyl trimethyl ammonium chloride reacted with cellulose in the presence of a catalytic amount of sodium hydroxide. A related approach has been the reaction of 2-chloroethyldiethyl amine with alkali cellulose. The product is then quaternized with methyl iodide in anhydrous alcohol. Finally, Uwatoko comments on a process where cellulose is reacted with a solution of sodium acid cyanamide at a concentration of 50-200 g/L at a pH in the range of 10-13 and temperature of 10.degree.-40.degree. C. for 4-24 hours.
McKelvey and Benerito, J. Appl. Polymer Sci. 11: 1693-1701 (1967), show the reaction of cellulose with a mixture of epichlorohydrin and a tertiary amine in the presence of aqueous sodium hydroxide.
The references cited are exemplary only since the preparation of cationic cellulose is not the subject of the present invention. The reader interested in a more detailed literature survey of cationic celluloses might refer to the present assignee's copending application, Ser. No. 507,366, filed June 24, 1983 now U.S. Pat. No. 4,505,775. This application, of which the present inventor is a coinventor, describes a very inexpensive and greatly simplified process for manufacturing a cationic cellulose. This is done by adding either a linear or partially crosslinked water soluble reaction product of epichlorohydrin and dimethylamine to an aqueous suspension of cellulose under alkaline conditions. The preferred concentrations of epichlorohydrin and dimethylamine will be approximately equimolar in proportion. Ammonia and primary aliphatic diamines serve to act as crosslinking agents for the reaction products. Further, their use increases the number of tertiary nitrogen atoms which may be quaternized to provide sites for positive charges. Up to 30 molar percent of the dimethylamine may be replaced by ammonia or the aliphatic diamine in the condensation process. In general, it is preferred that the molar percentage of the crosslinking material should be in the range of 10-20%. Preparation of suitable reaction products is described in U.S. Pat. No. 3,930,877 to Aitken.
It has been found that a cationic cellulose of the types described in the foregoing patent application can effectively bond a wide variety of anionic latices under the processing conditions normally used prior to the wet end of a paper machine. The products thus prepared have a wide variety of uses, particularly in areas where the fibers are later formed into air laid webs of various types.